Sublimation transfer method has been widely used for preparing printed articles since it can provide high-quality images of excellent transparency, and high reproducibility and high gradation of neutral tints, which are equivalent to the conventional full-color photographic images. As the printed articles, digital photographs, and ID cards which are used in various fields and known as identification card, driver's license card, membership card, and the like, are known.
In the case of image forming in accordance with the sublimation transfer method, a thermal transfer sheet that has a dye layer provided on one surface of a substrate is used. The dye layer of the thermal transfer sheet is superposed on a transfer receiving article such as a substrate for a card, and then, they are heated by a thermal head from the back face of the thermal transfer sheet so as to transfer many color dots of three or four colors onto the transfer receiving article, and thereby a printed article in which an image is formed on the transfer receiving article is obtained. According to the sublimation transfer method, since it is possible to control the transferring amount of the dye dot unit by dot unit with varying the amount of energy applied to the thermal transfer sheet, it is possible to perform a density gradation. Therefore, this method can provide a high quality image which is very vivid, and excels in the transparency, and the color reproducibility and the gradient of halftones, and which is comparable to full-color photograph image.
Although the sublimation transfer method is excellent in the formation of gradation image as mentioned above, the obtained print has disadvantages of an inferior light fastness and a poor durability, since the print is formed with dyes which each has a relatively low molecular weight, and they are different from the conventional printing ink which are made of pigments, and they does not have a vehicle. Therefore, recently, a method of forming a protective layer, in which a protective layer transfer sheet having a protective layer is superposed on the print formed by thermal transfer of sublimation dyes, then the protective layer is transferred onto the print by using a thermal head or heating roll or the like, thus, the protective layer is formed on the print, has been known. It is possible to improve the durability of the image by forming such a protective layer on the image.
For example, in Patent Literature 1, a protective layer transfer sheet which is provided with a protective layer comprising a curing type resin is disclosed. In Patent Literature 2, a protective layer transfer sheet which is provided with a protective layer mainly comprising solvent-insoluble organic particles and a binder resin is disclosed. According to the protective layer transfer sheets disclosed in these Patent literatures, it is said that the image (hereinafter, it is also referred to as “print”, occasionally.) is able to impart durability such as plasticizer resistance or the like. Further, other various attempts have been also made with respect to the protective layer transfer sheet for improving the plasticizer resistance of the image.
Incidentally, recently, there are some wishes to form a further image on the protective layer provided on the print by using various dyes. However, the protective layers which have been made various studies to date, for example, as are proposed in the Patent Literatures 1 and 2, are aimed to improve the durability, such as the plasticizer resistance or the like, that is usually required function for the protective layer, and thus it has not been studied at all for dyeing property against various dyes of the protective layer, and they have no dyeability. Therefore, it is not possible to perform image formation on any conventional protective layer. In addition, the protective layer has not been studied at all about releasability between the protective layer and the dye layer. Thus, when the image formation is carried out by superposing a dye layer on a protective layer which are known to date, it is not possible to sufficiently satisfy the releasability between the protective layer and the dye layer, it may cause problems such as an abnormal transfer where the dye layer is entirely transferred to the protective layer or breakage of the dye layer, as a result of closely sticking the dye layer on the protective layer.
Moreover, in the case that a protective layer integrated type thermal transfer sheet in which a protective layer and dye layers are arranged sequentially in parallel on a substrate across the surface of the substrate is used, and a continuous printing is performed so as to transfer the protective layer onto a print and to form an image on the transfer receiving article, if a certain trouble such as conveyance failure of a printer or the like may arise, it may cause an event that, on the protective layer transferred onto print, a dye layer which is used to form a next screen is overlapped. In such a case, if a releasability is not given to the protective layer, the dye layer of the protective layer integrated type thermal transfer sheet may be closely stuck on the protective layer which has been transferred at a previous screen, and as a result, the dye layer of the protective layer integrated type thermal transfer sheet may be broken out. Therefore, it becomes necessary to have a sufficient releasability for the protective layer in such a protective layer integrated type thermal transfer sheet.
Incidentally, when an image is simply formed on the protective layer, the as-is diversion of a binder resin which is contained in the dye receiving layer of the thermal transfer image-receiving sheet to the binder resin of the protective layer of the protective layer transfer sheet may be considered to be sufficient. Although the binder resin contained in the dye receiving layer of the thermal transfer image-receiving sheet has a dyeability, however, it is not have been considered for the plasticizer resistance, etc., at all, it cannot serve as a protective layer to protect the print. In other words, it can be said that the dyeability and the plasticizer resistance are in a trade-off relationship.